Answer: There are some pesticides registered safe for use on vegetable plants but without knowing what pest is causing the problem, you may want to hold off on using these chemicals. This is because anything that is non-specific to a particular pest will be a broad-spectrum insecticide and broad-spectrum insecticides will kill all insects, including bees (which you want to attract to the flowers of your veggies so they can transfer pollen). So, as long as they are only munching on the leaves of your plants, I'd suggest just putting up with the damage. If you can find or capture one of the critters then you can determine (or have nursery personnel identify it to determine) whether or not it is considered a destructive pest. If it is, you can use a pesticide specifically against that pest.
Melons and cukes take a long time to mature and there's not enough hot weather left for them to produce much of a crop. But, you might try planting radishes or carrots or even parsley in the empty spots. You can also grow leaf lettuces in those spots.
Or, if you plant marigolds, the are reputed to repel garden insects so you'd get pest control along with pretty flowers.
Whatever you plant, I wish you great success. I'm impressed that you've found a spot in which to garden in urban Chicago!!
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